cfi_flash: Add optional config register write to cfi-detection

This patch adds the possibility to (optinally) write to the
flash configuration register. The Intel style CFI chips support
such a register that can be used to configure the operation
mode to a non-default value.

This method will be used by the t3corp board, which needs to
configure the DS617 Xilinx flash for async read mode.

Signed-off-by: Stefan Roese <sr@denx.de>